The present invention relates to sheet transfer apparatus for multipress printing machines and in particular to apparatus for seizing the edge of the paper sheet on a rotatating transfer cylinder in order to turn the sheet over.
Multiprint printing machines such as multicolor presses require that the sheet be transferred between successive printing units to be impressed sequentially with different color and overlays and the like.
A transfer cylinder is employed to transfer the paper sheet from the first printing unit to the following printing unit. In practice it is often necessary to also print with several colors on both sides of the sheet during a single pass through the machine. In order to print a paper sheet on two sides it is necessary to turn the sheet while it is being transferred by the transfer cylinder to the adjacent print or intermediate transfer cylinders of the following printing unit. There are known sheet transfer cylinder whose diameters are made double the diameter of the adjacent cylinders. These transfer cylinders are provided with mechanical grippers which seize the leading edge of the paper sheet and hold the trailing edge of the sheet by a suction mechanism while turning the sheet. These suction devices effect the holding of the trailing edge of the sheet on the outer circumference of the transfer cylinder and are adjustably arranged at a given distance from the mechanical grippers which seize the leading edge of the paper sheet. This distance is adjustable according to the size of the paper sheet to be transferred.
A disadvantage of the above described device is in that the air distribution for the suction means is complicated, and as a result does not achieve a good printing register.
It has been attempted to provide sheet transfer cylinders which have suction devices for holding the trailing edge of the paper sheet with an air distributing system comprising special vacuum pressure tubing, a nozzle, a shaft and a carrying arm. The displacement and the adjustment of the suction devices on the circumference of the sheet transfer cylinder is made possible by means of a cam segment and a lever, on which electrically controlled means are arranged for turning of the carrying arm in the opposite directions. A disadvantage of this mechanism is in that the combined means is rather complicated and expensive in production, since there are two power sources; a vacuum air supplying the suction device and an electric source, connected to the carrying mechanism.
All hitherto known devices for holding the trailing edge of a paper sheet on the circumference of the sheet transfer cylinder employ the principle of vacuum suction devices. They must therefore be equipped with special vacuum supply sources and with driving units which increase the expense of the producer as well as of the printer.
It is an object of the present invention to provide apparatus for seizing and holding the trailing edge of the paper sheet on a transfer cylinder which overcomes the above noted disadvantages.
It is a further object of the present invention to provide apparatus for seizing and holding the trailing edges of the paper sheet on a transfer cylinder which is mechanical in nature and eliminates both suction devices and electrical controls.
It is a further object of the present invention to provide an improved simplified and inexpensive means for securing and holding paper sheets on a turning transfer cylinder.
These and other objects as well as numerous advantages are set forth in the following disclosure.